A. Technical Field
The present invention relates to a method for purification of a crude acrylic acid, which enables to efficiently eliminate aldehydes, particularly, furfural and/or acrolein, existing as impurities in the crude acrylic acid which is obtained by a vapor phase oxidation of propylene and/or acrolein.
B. Background Art
It is known that aldehyde such as furfural, acrolein, benzaldehyde, and so on and ketone such as acetone are by-produced as impurities in acrylic acid obtained by the vapor phase oxidation of propylene and/or acrolein. These impurities cause problems such as retardation of polymerization rate, decreasing of polymerization degree, coloring of polymers, and so on when acrylic acid is used as a raw material for a high molecular material such as water absorbing resins.
Generally the crude acrylic acid obtained by the vapor phase oxidation process is in general purified by distillation to eliminate impurities and, then, used in various applications. But it is difficult to separate aldehyde from acrylic acid because of close boiling point of acrylic acid and aldehyde such as furfural out of the above-mentioned impurities. For this reason, it is known to treat aldehyde with chemicals such as amine or hydrazine in order to raise boiling point of these aldehyde before distillation.
In order to achieve complete conversion of aldehyde with the aldehyde treatment chemical to form high boiling material, an excess amount of the aldehyde treatment chemical against an amount of aldehyde is used because of an equilibrium established in the reaction of aldehyde and the aldehyde treatment chemical. Also, in order to avoid the regeneration of aldehyde during distillation, distillation should be performed in the presence of a considerable amount of unreacted aldehyde treatment chemical. On the other hand the excess amount of the aldehyde treatment chemical induces polymerization of acrylic acid during distillation, which brings polymer deposit in a distillation column to cause deteriorating of heat transfer efficiency of a reboiler, interfering of distillation performance, and clogging in the distillation column, which may result in plant shut down.
It has been expected to establish the process of purifying acrylic acid to eliminate aldehyde efficiently without polymerization of acrylic acid, and several treatment methods have been proposed such as a method for distillation of the crude acrylic acid in the presence of hydrazine and sulfuric acid as described in JP-A-316027/1997 and a method for distillation of the crude acrylic acid at not more than 100° C. with hydrazine and copper dithiocarbamates as described in JP-A-228548/1995.
Since sulfuric acid and copper dithiocarbamates are used in addition to hydrazine as the aldehyde treatment chemical according to these methods, these material cause problems such as corrosion of metal on inner surface of equipment such as a column and piping. To solve this problem, a corrosion resistant material or anti-corrosive treatment are requested, which increases equipment cost and, hence, production cost. These methods are not industrially easy and economical one.